1. Field of the Invention
This invention relates to a bridging protection apparatus for an induction furnace which prevents the wall of the induction furnace from being damaged by bridging of a metal material to be melted in the furnace.
2. Description of Related Art
FIG. 14 is a diagram showing the configuration of a conventional induction furnace having a runout detection device. As shown in FIG. 14, the body of the induction furnace is made of a refractory material 5. To the outer periphery of the refractory material 5, a heat insulating material 6 is attached, so that the refractory material 5 and the heat insulating material 6 constitute a furnace wall. A coil 8 through which a large current can flow is wound around the outer periphery of the furnace body. When an alternating current (AC current) flows through the coil 8, a current is induced by electromagnetic induction in a metal material 3 to be melted and in a molten metal 2, thereby heating the metal material 3 and the molten metal 2. As a result, the metal material 3 melts and the temperature of the molten metal 2 rises. Since a large current of several thousands to tens of thousands amperes usually flows through the coil 8, the coil 8 is constructed of a hollow conductor provided with an inner passage for allowing cooling water to flow therethrough, thereby suppressing the temperature rise of the coil 8.
The refractory material 5 is gradually damaged when it is used for a long time. When the temperature of the molten metal 2 becomes extremely high, however, the refractory material 5 is damaged rapidly, thereby causing a runout accident, i.e., causing the molten metal 2 to leak out. Thus, the induction furnace is provided with a runout detection device which comprises first and second antennas 11 and 12 connected to each other through a detection power source 13 and a runout detector 14. The second antenna 12 is insulated with an insulating material 7 and disposed between the outer periphery of the furnace body and the coil 8. The first antenna 11 is disposed at the bottom of the furnace so that the first antenna 11 is brought in contact with the molten metal 2. When the molten metal 2 leaks out due to wear of the refractory material 5 to contact with the second antenna 12, therefore, a current flows from the detection power source 13 to the runout detector 14 and then through the second antenna 12, the molten metal 2 and the first antenna 11, and back to the detection power source 13. This current flow actuates the runout detector 14, thereby closing a runout alarm contact 14A resulting in an alarm that acts as notification in that theres runout.
FIG. 15 is a circuit diagram showing a conventional electric circuit of the induction furnace. In FIG. 15, an AC power is supplied from an AC power source 20 to an inverter unit 23 through a circuit breaker 21 and a transformer 22. The inverter unit 23 which comprises a rectifier 23R, a DC reactor 23L and a thyristor inverter 23S converts an input AC current into an AC current having a desired frequency. The converted AC current is supplied to the coil 8 of the induction furnace. A voltage detector 24, a voltage controller 26 connected thereto, a current detector 25, and a current controller 27 connected thereto are provided in the primary side of the transformer 22 in order to limit the levels of the voltage and current to be supplied to the coil 8 so as not to exceed a predetermined value.
FIG. 16 is a graph showing the operating characteristics of the induction furnace. The abscissa indicates a current, and the ordinate indicates a voltage. IL represents a current limiting value set by the current controller 27 shown in FIG. 15, and V.sub.L represents a voltage limiting value set by the voltage controller 26. At the beginning of its operation, the induction furnace exhibits the operating characteristics indicated by the straight line A in the graph. This means that a current can readily flow through the metal material 3 or the molten metal 2 because the temperature thereof is low. As the temperature rises, however, it becomes difficult for a current to flow through the metal material 3 or the molten metal 2. Thus, with the elapse of time, the operating characteristics gradually change into the state indicated by the straight line D.
As described above, in order to start the operation of the induction furnace, a metal material 3 is first fed into the furnace, and then an AC current is allowed to flow through the coil 8, so that the metal material 3 is heated to melt into a molten metal 2 by electromagnetic induction. When the temperature of the thus obtained molten metal 2 rises, the metal material 3 is further supplied through the top of the furnace into the molten metal 2. This causes the temperature of the molten metal 2 to be temporarily lowered. When the temperature of the molten metal 2 rises again by electromagnetic induction, the metal material 3 is further supplied through the furnace top into the molten metal 2. In this process, the following are repeated. As described above, the operating characteristics shown in the graph of FIGS. 16 first change from the state of the straight line A to that of the straight line D. When the metal material 3 is supplied into the molten metal 2 to lower the temperature thereof, the operating characteristics return to the state of the straight line A, and then gradually change again into the state of the straight line D with an increase in the temperature of the molten metal 2.
FIG. 17 is a diagram showing the configuration of the induction furnace in which bridging of a metal materials has occurred. As shown in FIG. 17, the lumps of the metal material 3 supplied through the furnace top have been intertwined to keep them from falling into the molten metal 2, i.e., bridging of the metal material 3 has occurred. In the case of such bridging, the electric power supplied to the induction furnace is supplied to the molten metal 2, but it is not supplied to the metal material 3 in the state of bridging. Thus, the temperature of only the molten metal 2 rises, sometimes to 2,000.degree. C. in the worst case. At such a high temperature, the refractory material 5 is rapidly worn, thereby causing the danger of a runout accident. When such runout causes the coil 8 to break, the leaked molten metal 2 may come into contact with the cooling water in the coil 8, thereby causing the danger of a steam explosion. Even if a runout accident does not occur, the metal material 3 in the state of bridging may collapse and fall into the molten metal 2 heated to an extremely high temperature, whereby a gas may be rapidly generated to blow up the molten metal 2. In the case of a closed induction furnace, there is danger of explosion due to the pressure of the generated gas.
The only available way of finding out whether bridging of the metal material 3 has occurred or not is the visual inspection of the status of the furnace. However, it is difficult and dangerous to constantly perform the visual inspection of the status of bridging in the poor work environment. Furthermore, in the case of a closed-type induction furnace with its furnace top closed, it is impossible to check the status of bridging by visual inspection.
FIG. 18 is a graph showing the operating characteristics of the induction furnace in which bridging has occurred. In the same manner as in the graph of FIG. 16 described above, the abscissa indicates a current, the ordinate indicates a voltage, I.sub.L represents a current limiting value, and V.sub.L represents a voltage limiting value. In the case where bridging has occurred in the induction furnace, the metal material 3 does not fall into the molten metal 2, so that the temperature of the molten metal 2 does not decrease. Therefore, unlike the above-described graph of FIG. 16, the operating characteristics are unchanged or remain in the state indicated by the straight line D.